In situ studies of diffusion and crystallization processes in thin ITO films by temperature and time resolved grazing incidence X-ray diffractometry

M. Quaas ¹, H. Wulff ¹, H.Steffen ², R. Hippler ²

¹Institute of Chemistry and Biochemistry , University of Greifswald, Soldmannstraße 23, D-17489 Greifswald, Germany

²Institute of Physics, University of Greifswald, Domstraße 10a, D.17489 Greifswald, Germany

Studies of diffusion and crystallite growth in thin films have attracted wide attention due to the increased use of multilayered thin film structures in modern electronic devices. Particularly with regard to the time stability of such devices, the investigation of phase transitions or diffusion processes are of special interest. In situ high temperature grazing incidence X-ray diffractometry (HT-GIXD) is well suited, but so far rarely used for characterizing the kinetic parameters of such processes. We present both a quantitative description of oxygen diffusion into plasma enhanced deposited metallic In/Sn films and of the crystallite growth of indium tin oxide thin films.

Tin-doped indium oxide (ITO) films were deposited on Si(100) substrates without external heating by means of dc planar magnetron sputtering. A metallic In/Sn (90/10) target and an argon/oxygen gas mixture were used. The flow of the reactive gas oxygen was varied between 0 and 2 sccm. Bias voltages between 0 and –100V were used. With increasing oxygen flow, the film structure and composition changes from crystalline metallic In/Sn to amorphous ITO. Oxygen diffusion into metallic In/Sn films and crystallite growth of ITO films were investigated by in situ HT-GIXD at temperatures ranging from 100 to 300 °C and a vacuum of 5*10^-3 mbar.

The ITO formation is determined by two processes: the diffusion of oxygen into the metallic grains and a fast crystallization process. Kinetic parameters for both processes were derived. A model was developed which allows the determination of the diffusion coefficient D from the time dependence of the integral intensity of the ITO-X-ray reflection. This mathematical model incorporates the following physical basics:

A thin film of the material n (In/Sn) converts into a crystalline phase m (ITO) in a diffusion limited process. The intensity of an X-ray reflection of phase m in the film at time t can be calculated using Eq. (1)

(1)

with B_m the theoretical intensity factor, K₀ the apparatus constant, I₀ the intensity of the incident X-ray beam and A the irradiated sample area. The ratio C(x,t)/C₀ is the fraction of phase m in the depth x. The absorption correction factor A_m depends on the phase composition: with

and a geometry factor z. The time dependence of C(x,t)/C₀ is given by Fick’s second law

.

The amount of amorphous ITO in the as-deposited films is considered by a factor f.

By integration Eq.(1) over the film thickness d with the above mentioned assumption, the intensity of a reflection of phase m follows:

(2).

A detailed description of the model is given in [1,2]

The diffusion coefficient depends on the applied bias voltage but it is not influenced by the oxygen flow during film deposition. From the temperature dependence of D the activation energies for the diffusion process can be calculated [3]. For T<150°C the activation energy depends on the applied bias voltage. For T>150°C the D values still differ with the bias voltage but the activation energy seems to be independent of the deposition conditions.

The crystallization of amorphous ITO can be understood by application of the Johnson-Mehl-Avrami theory [3]: with y_m normalized intensity, t the annealing time, n the reaction order and 1/t₀ the rate constant. For our films we get n-values between 2.4 and 3, i.e. a two dimensional crystallization process [4].

Our experimental results show that the application of in situ GIXD measurements is well suited to investigate temperature induced processes. This method establishes a wide range of analytical possibilities.

Acknowledgement

This work has been supported by the German Research Foundation under SFB198/A11.

[1] H.Wulff, M.Quaas, H.Steffen, R. Hippler, Thin Solid Films 377-378 (2000) 418-424

[2] H.Steffen, H. Wulff, M.Quaas, T.M.Tun, R.Hippler, Acta Phys. Slov. 50(2000) 667-673

[3] M. Avrami, J.Chem. Phys. 8 (1949) 212

[4] Y. Shigesato, D.C. Paine, Thin Solid Films 238 (1994) 44